Polypeptides and peptides have become increasingly important agents in a variety of applications, including industrial applications and use as medical, therapeutic and diagnostic agents. However, in certain physiological states, such as inflammatory states (e.g., COPD) and cancer, the amount of proteases present in a tissue, organ or animal (e.g., in the lung, in or adjacent to a tumor) can increase. This increase in proteases can result in accelerated degradation and inactivation of endogenous proteins and of therapeutic peptides, polypeptides and proteins that are administered to treat disease. Accordingly, some agents that have potential for in vivo use (e.g., use in treating, diagnosing or preventing disease) have only limited efficacy because they are rapidly degraded and inactivated by proteases.
Protease resistant polypeptides provide several advantages. For example, protease resistant polypeptides remaining active in vivo longer than protease sensitive agents and, accordingly, remaining functional for a period of time that is sufficient to produce biological effects. A need exists for improved methods to select polypeptides that are resistant to protease degradation and also have desirable biological activity.
TNFR1
TNFR1 is a transmembrane receptor containing an extracellular region that binds ligand and an intracellular domain that lacks intrinsic signal transduction activity but can associate with signal transduction molecules. The complex of TNFR1 with bound TNF contains three TNFR1 chains and three TNF chains. (Banner et al., Cell, 73(3) 431-445 (1993).) The TNF ligand is present as a trimer, which is bound by three TNFR1 chains. (Id.) The three TNFR1 chains are clustered closely together in the receptor-ligand complex, and this clustering is a prerequisite to TNFR1-mediated signal transduction. In fact, multivalent agents that bind TNFR1, such as anti-TNFR1 antibodies, can induce TNFR1 clustering and signal transduction in the absence of TNF and are commonly used as TNFR1 agonists. (See, e.g., Belka et al., EMBO, 14(6):1156-1165 (1995); Mandik-Nayak et al., J. Immunol, 167:1920-1928 (2001).) Accordingly, multivalent agents that bind TNFR1, are generally not effective antagonists of TNFR1 even if they block the binding of TNFα to TNFR1.
The extracellular region of TNFR1 comprises a thirteen amino acid amino-terminal segment (amino acids 1-13 of human TNFR1; amino acids 1-13 of mouse TNFR1), Domain 1 (amino acids 14-53 of human TNFR1; amino acids 14-53 of mouse TNFR1), Domain 2 (amino acids 54-97 of human TNFR1; amino acids 54-97 of mouse TNFR1), Domain 3 (amino acids 98-138 of human TNFR1; amino acid 98-138 of mouse TNFR1), and Domain 4 (amino acids 139-167 human TNFR1; amino acids 139-167 of mouse TNFR1) which is followed by a membrane-proximal region (amino acids 168-182 of human TNFR1; amino acids 168-183 mouse TNFR1). (See, Banner et al., Cell 73(3) 431-445 (1993) and Loetscher et al., Cell 61(2) 351-359 (1990).) Domains 2 and 3 make contact with bound ligand (TNFβ, TNFα). (Banner et al., Cell, 73(3) 431-445 (1993).) The extracellular region of TNFR1 also contains a region referred to as the pre-ligand binding assembly domain or PLAD domain (amino acids 1-53 of human TNFR1; amino acids 1-53 of mouse TNFR1) (The Government of the USA, WO 01/58953; Deng et al., Nature Medicine, doi: 10.1038/nm1304 (2005)).
TNFR1 is shed from the surface of cells in vivo through a process that includes proteolysis of TNFR1 in Domain 4 or in the membrane-proximal region (amino acids 168-182 of human TNFR1; amino acids 168-183 of mouse TNFR1), to produce a soluble form of TNFR1. Soluble TNFR1 retains the capacity to bind TNFα, and thereby functions as an endogenous inhibitor of the activity of TNFα.